Method and apparatus for separating isotopes

ABSTRACT

The improved isotope separator comprises a vacuum vessel, a plasma generator located substantially in the center of the vacuum vessel, an electrode bounded by a hyperboloid of one sheet and a pair of electrodes bounded by a hyperboloid of two sheets, said electrodes being located within the vacuum vessel in such a way as to surround the plasma generator, a power source for supplying said electrodes with a fixed voltage and a pulsating voltage, and magnetic field generating means located outside the vacuum vessel. The apparatus is implemented by a method for isotope separation that achieves high separation factor per stage (process), that enables the process throughput to be increased with ease and which yet is applicable to the isotopic separation of many elements.

This application is a division of application Ser. No. 08/607,467, filedFeb. 27, 1996, now U.S. Pat. No. 5,653,854.

BACKGROUND OF THE INVENTION

This invention relates to the separation of isotopes such as those ofuranium having different masses. In particular, the invention relates toa method by which an ionized substance can be separated into respectiveisotopes in accordance with the mass difference in the combination of amagnetic field and an electric field composed of a pulsating voltagesuperposed on a fixed voltage, as well as an apparatus for implementingthe method.

The isotope separation technology has so far been largely developed tomeet the need for enriching uranium-235 as a nuclear reactor fuel.Recently, it has been proposed that only the isotopes of metal elementshaving low activation characteristics that are relatively preferred forstructural materials of nuclear fusion reactors, large-scaleaccelerators, etc. should be separated for use. This is because theselective separation and use of the required isotopes facilitates themaintenance of fusion reactors and large-scale accelerators and achievesconsiderable reduction in radioactive wastes to be disposed of. Toobtain these benefits, it is required to develop a technique that iscapable of separating the isotopes of metal elements at low cost.

Methods that have heretofore been used to separate uranium's isotopesand other atoms or molecules having small mass differences includegaseous diffusion, centrifugal separation, chemical approach(adsorption), mass separation in an electromagnetic field, and selectiveionization with laser beams.

The gaseous-diffusion, centrifugal separation and chemical process sharethe common problem of an extremely low separation factor per stage(process), requiring a number of stages to be implemented in practicalapplications.

The mass-separation process using an electromagnetic field achieves highseparation factor but, on the other hand, due to its operating principlewhich is based on single-particle loci in high vacuum, the throughput ofthe process is extremely small. The process also involves instrumentalproblems as exemplified by the need to limit the initial velocity anddirection of ions to certain ranges for attaining the necessaryresolution or to provide beam limiting slits.

The selective ionization method with laser beams, as it is applied touranium separation, comprises illuminating a uranium atomic beam withdye laser light to achieve selective excitation of uranium-235 while, atthe same time, other laser light is applied to further ionize theuranium-235. This method has met with success in uranium separation;however, with the scarcity of data accumulated for the isotopicseparation of other elements; the method has no general applicability.

SUMMARY OF THE INVENTION

The present invention has been accomplished under these circumstancesand has as an object providing a method for isotope separation thatachieves high separation factor per stage (process), that enables theprocess throughput to be increased with ease and which yet is applicableto the isotopic separation of many elements.

Another object of the invention is to provide an apparatus suitable forimplementing the method.

According to the first aspect of the invention, there is provided amethod for separating a substance of interest into isotopes of differentmasses in an electromagnetic field, characterized in that the substancein an ionized form is subjected to both a magnetic field parallel to theZ axis and an alternating electric field U defined by the followingequation (1) which is created by a fixed voltage and a pulsating voltagethat are applied to an electrode bounded by a hyperboloid of one sheet(in a pincushion form) and a pair of electrodes bounded by a hyperboloidof two sheets (in the form of the combination of an inverted pileus anda non-inverted pileus), such that the ions lighter than a specifiedcritical mass M_(c) are transported in the axial (Z) direction asdistinguished from the heavier ions which are transported in the radial(r) direction:

    U=1/2·k.sub.0 (1-A cos 2θ))(-r.sup.2 +2Z.sup.2)(1)

U : alternating electric field

k₀ : constant

A : constant

θ: time function

: radial distance

Z : axial distance

In short, the isotope separation method of the invention, which uses anelectromagnetic field to separate a substance of interest into isotopeshaving different masses, is based on the novel finding that in the spaceof a special electromagnetic field that is expressed by cylindricalcoordinates, ions of like species generated in a wide region of thatspace can selectively be transported in the axial or radial directionirrespective of the initial velocity of the ions or the direction inwhich they start to travel.

This method allows the ions lighter than a specified critical mass M_(c)to be transported in the axial direction as distinguished from theheavier ions that are transported in the radial direction.

The method can be implemented with an apparatus for isotope separationthat comprises a vacuum vessel, a plasma generator located substantiallyin the center of said vacuum vessel, an electrode bounded by ahyperboloid of one sheet and a pair of electrodes bounded by ahyperboloid of two sheets, said electrodes being located within saidvacuum vessel such a way as to surround said plasma generator, a powersource for supplying said electrodes with a fixed voltage and apulsating voltage, and magnetic field generating means located outsidesaid vacuum vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section of the general composition of anisotope separator according to an embodiment of the invention;

FIG. 2 is a cross section taken on the line A--A' of FIG. 1 showingdetails of the surface. FIG. 2A show part of the pincushion electrodeused in the embodiment of FIG. 2;

FIG. 3 is a graph showing the movement of ions relative to the positionsof a pair of electrodes,one being in the form of an inverted pileus andthe other in the form of a non-inverted pileus, in the embodiment;

FIG. 4 is a graph showing the movement of ions relative to thepincushion electrode in the embodiment; and

FIG. 5 is a longitudinal section of an isotope separator according toanother embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The vacuum vessel to be used in the invention is in no way limited but,typically, it has a generally cylindrical form which conforms to thespace of an applied electromagnetic field and for the purpose of stableion generation, the vessel is advantageously adapted to be capable ofcreating a high-vacuum atmosphere by being coupled to a pump-downapparatus via an exhaust port. The construction material of the vacuumvessel is determined in consideration of various factors including themechanical strength and workability; metals such as stainless steel andaluminum alloys are advantageous.

The plasma generator to be used in the invention functions as a plasmasource and is located within the vacuum vessel. As in the case of thevacuum vessel, the plasma generator is not limited to any particulartype but, advantageously, it comprises a center arc discharge electrodesheld between two arc discharge electrodes, all electrodes being formedof the substance to be separated into isotopes.

The electrode bounded by a hyperboloid of one sheet and which is used inthe isotope separator of the invention is typically composed of anelectrode in a pincushion form. This electrode combines with a pair ofelectrodes bounded by a hyperboloid of two sheets such as to form analternating electric field defined by the equation (1).

The electrode bound by a hyperboloid of one sheet is intended to trapheavy ions by causing them to be condensed on the electrode surface. Tothis end, it is advantageous that the side of the principal electrodefacing the isotope separating space has a number of fins detachablyembedded in positions generally parallel to the Z axis. The use of anelectrode having this structure is favorable for the purposes of theinvention in that the neutral particles coming straight from the plasmagenerator and the ions rotating about the Z axis can be deposited indifferent areas of the electrode and, hence, can be recovered in asimple manner.

The electrode bounded by a hyperboloid of one sheet is located withinthe vacuum vessel in such a way as to surround the plasma generator. Toachieve an improved efficiency in the separation and trapping of heavyions, this electrode is advantageously located in a radial positiongenerally greater than the maximum ion amplitude. Specifically, theradial position of the electrode is preferably within the range from 0.8to 5.0 times, more preferably from 1.0 to 1.5 times, the maximum ionamplitude. If the radial position of the electrode is less than 0.8times or more than 5.0 times the maximum ion amplitude, the efficiencyin the separation and trapping of heavy ions fails significantly or theapparatus becomes unduly bulky. If the radial position of the electrodebounded by a hyperboloid of one sheet is within the range from 1.0 to1.5 times the maximum ions amplitude, better efficiency is assured forthe separation and trapping of heavy ions and the apparatus has areasonable size.

The pair of electrodes bounded by a hyperboloid of two sheets and whichare used in the isotope separator of the invention are intended to traplight ions by causing them to be condensed on the electrode surfaces.They consist typically of the combination of an inverted and anon-inverted pileus and combine with the electrode bounded by ahyperboloid of one sheet such as to form an alternating electric fielddefined by the equation (1).

As in the case of the electrode bound by a hyperboloid of one sheet, itis advantageous that the side of each principal electrode facing theisotope separating space has a number of fins detachably embedded inpositions generally parallel to the radial direction. The electrodessuffice to be located in such a way as to form an alternating electriccurrent; specifically, it is advantageous that the electrodes arelocated within the vacuum vessel in such a way as to hold the plasmagenerator therebetween.

In consideration of separation and trapping efficiency, the electrodesare advantageously located in vertical positions that are generallygreater than the maximum ion amplitude in the axial (Z) direction.Specifically, the vertical positions of the electrodes are preferablywithin the range from 0.8 to 5.0 times, more preferably from 1.0 to 1.5times, the maximum ion amplitude. If the vertical positions of theelectrodes are less than 0.8 times or more than 5.0 times the maximumion amplitude, the efficiency in the separation and trapping of lightions fails significantly or the apparatus becomes unduly bulky. If thevertical positions of the electrodes bounded by a hyperboloid of twosheets are within the range from 1.0 to 1.5 times the maximum ionamplitude, better efficiency is provided for the separation and trappingof light ions and the apparatus has a reasonable size.

The magnetic field generating means for use in the isotope separator ofthe invention also is not limited in any particular way and customarycoils may be employed. The positioning of the magnetic field generatingmeans also is not limited in any particular way but considering theefficiency of ion trapping, it is advantageously located outside thevacuum vessel.

It should be noted that various modifications of the invention can beimplemented without departing from its scope and spirit.

It should also be noted that the substance to be separated into isotopesis in no way limited to uranium and metal ions, provided that they arecondensable or can be trapped by adsorption on collecting plates.

The following examples are provided for the purpose of furtherillustrating the invention but are in no way to be taken as limiting.

EXAMPLE 1

FIG. 1 is a longitudinal section of the general composition of theapparatus for isotope separation according to an embodiment of theinvention. As shown, a cylindrical vacuum vessel 21 has an exhaust port22 through which high vacuum is drawn by operating a pump-down apparatus23. The vacuum vessel 21 is surrounded by coils 24 that form a magneticfield in the axial direction of the cylinder which is the geometryassumed by the vessel 21. Located within the vacuum vessel 21 are anelectrode 25 bounded by a quadric surface in a pincushion form that iscoaxial with the center of the vessel, as well as an upper electrode 26abounded by a quadric surface in an inverted pileus form and a lowerelectrode 26b also bounded by a quadric surface but in a non-invertedpileus form. The electrodes 26a and 26b are also coaxial with the centerof the vacuum vessel 21. The electrode 25 is supplied with the necessaryvoltage from a power source (not shown) via a current lead-in terminal27 whereas the electrodes 26a and 26b are supplied with the necessaryvoltage via respective current lead-in terminals 28a and 28b.

A plasma generator 29 in a cylindrical form of a specified diameter isinserted in and coaxial with the center of the vacuum vessel 21. In thecase shown in FIG. 1, the plasma generator 29 comprises a center arcdischarge electrode 30a held between two arc discharge electrodes 30b,each electrode being composed of the substance to be separated. Needlessto say, various other modifications may be conceived for the plasmagenerator. The electrodes that constitute the plasma generator aresupplied with a current through electric wires (not shown) providedalong equipotential surfaces that are formed in the isotope separatingspace 31 within the vacuum vessel 21.

The electrode 25 having a quadric surface in a pincushion form providesa hyperboloid of one sheet whereas the electrodes 26a and 26b having apair of quadric surfaces consisting of an inverted and a non-invertedpileus provide a hyperboloid of two sheets. These electrodes are soconfigured that when the center of the apparatus is taken as the origin,with the axial and radial directions being designated the Z and rdirections, respectively, a static electric field U' expressed by thefollowing equation (2) can be created within the isotope separatingspace 31

    U'=1/2·k'(-r'.sup.2 +2Z'.sup.2)                   (2)

U': static electric field

k': constant

r': radial distance

Z': axial distance

A discussion will now be made of the case where uranium or othersubstance are isotopically separated or enriched by the systemconfiguration shown in FIG. 1. The process starts with operating thepump-down apparatus 23 to create high vacuum in the interior of thevacuum vessel 21. Subsequently, the coils 24 are energized to create aspecified magnetic field in the axial direction while, at the same time,electrodes 25, 26a and 26b are each supplied with a specified voltage.

The supplied voltage causes an alternating electric field with smallpulsations to be created within the isotope separating space 31according to the equation (1). With the space of the electromagneticfield being thusly placed in the proper condition, the plasma generator29 is actuated. In Example 1 under consideration, the arc dischargeelectrodes 30a and 30b are supplied with a sufficient voltage to cause asustained discharge. The geometries of the discharge electrodes and thevoltage to be applied thereto are determined in such a way as to assurethat the space of the electromagnetic field will now be greatlydisturbed by the insertion of those electrodes.

Plasma generator 29 produces monovalent atomic (molecular) ions,electrons, neutral atoms (molecules), etc. from the substance to beseparated into isotopes. Ions and neutral particles, coming out of theplasma generator 29, will flow into the isotope separating space 31 atvarious initial velocities in different directions. If the substance tobe separated is natural uranium, ²³⁵ U⁺, ²³⁸ U⁺, ²³⁵ U⁰, ²³⁸ U⁰ andother isotopes will be produced and flow into the isotope separatingspace 31.

The light ions of ²³⁵ U⁺ flowing into the isotope separating space 31will rotate around the Z axis at high velocity. In the radial (r)direction, these light ions vibrate at varying amplitudes in the spacebetween the plasma generator 29 and the pincushion electrode 25 but inthe axial (Z) direction, the vibration of those ions will diverge withthe maximum amplitude increasing with time (the progress of rotation).Hence, the light ions of ²³⁵ U⁺ will impinge on the electrodes 26consisting of the combination of an inverted and a non-inverted pileus,on which they are condensed.

In the axial (Z) direction, the heavy ions of ²³⁸ U⁺ cyclically vibrateat varying amplitudes in the space between the electrodes 26a and 26bbut in the radial (r) direction, the vibration of those ions willdiverge with the maximum amplitude increasing with time (the progress ofrotation). Hence, the heavy ions of ²³⁸ U⁺ will impinge on thepincushion electrode 25, on which they are condensed.

On the other hand, ²³⁵ U⁰ and ²³⁸ U⁰ which are neutral particles are notaffected by the electromagnetic field, so they will travel straight fromthe plasma generator 29 and impinge on either of the electrode surfacesthat are on the line of sight from the plasma generator and on whichthey are condensed.

An electrode structure of the type shown in FIG. 2 and FIG. 2A may beused as means for distinguishing between the ions that rotate about theZ axis such as to reach the electrode with an increasing vibrationalamplitude and the neutral particles that travel straight from the plasmagenerator toward the electrode without being separated isotopically.FIG. 2 is a cross section taken on the line A--A' of FIG. 1 and showsdetails of the surface and 2A illustrates part of the pincushionelectrode 25 which also serves as a particle collecting plate. As shownin FIG. 2A, the principal electrode 41 in a pincushion form has a numberof fins (fin-shaped plates) 42 detachably embedded on the side facingthe isotope separating space in positions that are parallel to the Zaxis. This electrode design ensures that the neutral particlestravelling straight from the plasma generator 29 are deposited ondifferent locations than the ions rotating about the Z axis.

The same surface structure may be adopted by the electrode 26a in theform of an inverted pileus and the electrode 26b in the form of anon-inverted pileus and this again allows the ions to be deposited ondifferent locations than the neutral particles.

The substances such as the isotopes of uranium that have been depositedon electrodes 25, 26a and 26b in a separated or enriched form may berecovered by the following procedure; the electrodes are taken out ofthe vacuum vessel, with the fins 42 being optionally detached from theprincipal electrode 41, and the separated substances are chemicallydissolved or otherwise treated for recovery.

FIGS. 3 and 4 illustrate the loci ²³⁵ U⁺ and ²³⁸ U⁺ emerging from theplasma generator 29 will respectively describe during their flight inthe isotope separating space 31, with the critical mass M_(c) being setat 237.

FIG. 3 shows schematically the loci of ions travelling in the axial (Z)direction and the solid lines indicate the time profile (or envelope) ofthe maximum amplitude of ion vibrations. As is clear from FIG. 3, thevibration of ²³⁸ U⁺ is confined within a certain space whereas thevibration of ²³⁵ U⁺ diverges with the amplitude progressively increasingwith time until its ions reach the positions where the electrodes 26aand 26b are installed. In the case shown in FIG. 3, the electrodes 26aand 26b are located farther away from the maximum amplitude of thevibration of ²³⁸ U⁺, i.e., at least about 8 times as high as theion-generating element in the plasma generator 29.

FIG. 4 shows schematically the loci of ions travelling in the radial (r)direction and the solid lines indicate the time profile (envelope) ofthe maximum amplitude of ion vibration. As is clear from FIG. 4, thevibration of ²³⁵ U⁺ is confined within a certain space whereas thevibration of ²³⁸ U⁺ diverges with the amplitude progressively increasingwith time until its ions reach the positions where the pincushionelectrode 25 is installed. In the case shown in FIG. 4, the electrode 25is located in a radial position that exceeds the maximum amplitude ofthe vibration of ²³⁵ U⁺, i.e., at least about 11 times as great as theradius of the plasma generator 29.

EXAMPLE 2

FIG. 5 is a longitudinal section of an isotope separator according toanother embodiment of the invention, in which the cylindrical vacuumvessel has a group of annular electrodes formed as an integral part forcreating a specified electric field in the isotope separating space. Theapparatus of this second embodiment features compactness.

The cylindrical vacuum vessel 51 is coupled to a pump-down apparatuswhich draws high vacuum via an exhaust port (not shown). The vessel 51is surrounded by coils 52 that form a magnetic field in the axialdirection of the cylinder which is the geometry assumed by the vessel51. The inner surface of the vacuum vessel 51 is coated with an electricinsulator 53 such as Teflon, which spaces apart annular electrodes 54provided in the inner surface of the vessel 51. Each of the annularelectrodes is supplied with a specified voltage.

The voltage to be applied to the annular electrodes is so set that it isequivalent to the application of specified values of a fixed voltage anda pulsating voltage to the pincushion electrode and a pair ofelectrodes, one being in the form of an inverted pileus and the other inthe form of a non-inverted pileus (see Example 1), whereby analternating electric field U expressed by the equation (1) and havingsmall pulsations is created within the isotope separating space. Statedmore specifically, each of the annular electrodes is supplied with avoltage that is equal to what would be sensed by those annularelectrodes if the cylinder fitted with them were virtually immersed inthe alternating electric field that is formed when specified values of afixed voltage and a pulsating voltage are applied to the pincushionelectrode and the pair of electrodes, one being in the form of aninverted pileus and the other in the form of a non-inverted pileus.

Two plasma generators 55a and 55b are provided along the Z axis in thecenter of the vacuum vessel 51, and an atomic (molecular) beam source56a (or 56b) capable of supplying the atom of the substance to beseparated (or the molecular containing said substance) is provided abovethe plasma generator 55a (or below the plasma generator 55b). The plasmagenerator 55a (or 55b) is composed of an annular tungsten filament 57a(or 57b) and an equipotential electrode 58a (or 58b). The hot electronsradiated from the filament 57a (or 57b) act effectively on the atoms (ormolecules) supplied from the atomic (molecular) beam sources in themagnetic field (to cause PIG discharge), thereby enabling more ions tobe generated.

Thus, the present invention provides a method and an apparatus forisotope separation that achieve high separation factor per stage(process), that enables the process throughput to be increased with easeand which yet is applicable to the isotopic separation of many elements.

What is claimed is:
 1. An apparatus for isotope separation thatcomprises a vacuum vessel, a plasma generator located substantially inthe center of said vacuum vessel, an electrode bounded by a hyperboloidof one sheet, a pair of electrodes bounded by a hyperboloid of twosheets, said electrodes being located within said vacuum vessel in sucha way as to surround said plasma generator, a means for supplying saidelectrodes with a fixed voltage and a pulsating voltage to generate analternating electric field, and magnetic field generating means locatedoutside said vacuum vessel for generating a magnetic field parallel tothe z-axis superimposed upon the electric field.